This invention relates to optical recording and more particularly to optical information storage systems utilizing multiple lens scanners.
Optical recording, and in particular optical digital recording is well known and established within the art. Essentially, optical recording is used for high density storage of binary information and therefore is a technique of digital recording. Essentially, each information bit is stored on a reocrd as a discrete direct image. This is contrasted to other techniques which utilize halographic transform images. Storage densities utilizing optical digital recording techniques are exceptionally high. They are facilitated by the use of short focal lenses and mechanical scanning devices. There are two basic categories in the ways that optical scanning is performed. One category is that the recording material is rotated while either the recording material or the short focal lens is translated. It has been reported, for example, in SPIE Proceeding Vol. 123 edited for Optical Storage Materials and Methods, p. 104 (1977), "An Optical Disc Data Recorder", R. McFarlane et al that a total information capacity of 10.sup.10 bits may be recorded on a 30 cm disc which is equivalent to that recorded on 60 magnetic tapes.
Another category of scanning method is that the short focal lens or lenses are rotated by being mounted on a rotating scanning disc while either the rotating scanning disc or the recording material is translated. It has been reported, for example in Optical Engineering, Vol. 15, No. 1, January/February 1976, "Optical Digital Recording", Russell et al that data densities in the range of 5.times.10.sup.7 bits/cm.sup.2 with data rates of 3.times.10.sup.7 bits/sec. have been demonstrated. As reported, this data storage technique allows for storage on an area of approximately 1 in.sup.2 of an optical record, all of the information contained in a typical reel of computer tape, such as 2500 feet in length with a reading rate of 1600 BPI.
Various digital encoding techniques are used in optical digital recording, a typical one being the differential pulse code modulation format (PCM) which is used for color television program material. Accordingly, optical digital recording finds one area of commercial feasibility in consumer prerecorded television programing. In such an area, a video record would be compact in size, simple to operate, and will offer low cost when compared with contemporary video tape recording systems.
A variety of scanner techniques are known and have been demonstrated in this technology. Typical of prior art optical digital recording systems are those described in a group of patents of J. T. Russell, U.S. Pat. Nos. 3,501,586, 3,624,284, 3,795,902 and 3,806,643. In that typical system utilized for digital recording or digital playback, a laser source generally used is a beam passing through a modulator and then reflected off a mirror before being directed onto a distributor unit. The distributor is located relative to a scanner wheel having a number of microscope objective lenses. In the system described in the prior art reported by Russell, the record element is held stationary during both recording and playback and the scanner element of both rotates and is translated across the stationary digital record.
As indicated, a distributor is utilized which is a four face mirror located at one end of the scanner shaft. The function of the mirror is to direct an input light beam to a proper rotating objective during an active part of the scan across the record.
The microscope objective lens focuses the input laser light beam precisely onto the record where by means of fiber optics disposed on the opposite side coupled to a detector readout occurs. That is, in one technique described in the prior art readout occurs by which diverging light transmitted through the record is collected by a bundle of fiber optics which are then fed to a photomultiplier tube. There are, however, other ways of reading out the record.
In another technique, a large area of the record can be illuminated and the optics are arranged to image the data field only onto a masked area which defines an individual field to be read. This technique is not deemed to be as efficient as the first one since the illuminating light energy is not efficiently used. In the second technique, by use of focusing with the microscope objectives a spot approximately the size of 1 bit is illuminated.
Although these prior art optical digital recording and playback systems have demonstrated a proof of concept efficiency, nevertheless problems exist which deteriorate the accuracy of data recovered during playback. One problem is that there exists a time period during the scanning operation when two objective lenses on the rotating scanner wheel will be simultaneously illuminated. This period occurs when a first lens on the scanner is reading the end of one track while, a second or following lens mounted on the scanner wheel is beginning its scan of a subsequent track. Accordingly, a light collector will tend to collect data from both locations and interference of readout will exist at this time portion. The problem of interference at the end portions of a digital data track during the time period when two microscope objectives illuminate portions of the record remains an area of continuing research in this technology.